This invention relates to optical amplifiers and, more particularly, to lightwave systems and receivers utilizing such amplifiers.
The distance over which an optical signal can be transmitted on an optical fiber is a function of the loss and dispersion characteristics of the fiber, the spectral and power characteristics of the laser, and the number of optical amplifiers or electrical repeaters in the transmission path between transmitter and receiver. In conjunction with system specifications, these characteristics determine the spacing between the amplifiers or repeaters. Of course, it is well known that an optical amplifier amplifies the optical signal directly at optical frequencies, without converting it to a corresponding electrical signal, but an electrical repeater first converts the optical signal to an electrical analog, then regenerates the electrical signal before retransmitting it at an optical frequency.
A dispersion-limited signal (e.g., a signal matched to the 1.5 .mu.m loss minimum of present fiber) typically requires regenerative repeating to increase the transmission distance and single longitudinal mode lasers to reduce dispersion effects, whereas a loss-limited singal (e.g., a signal matched to the 1.3 .mu.m dispersion minimum of present fiber, or a single longitudinal mode signal at the 1.5 .mu.m loss minimum) can use the much simpler technique of direct optical amplification.
It has been theoretically predicted that semiconductor laser chips, modified to function as optical amplifiers, can increase the transmission distance between regenerators in fiber optic transmission systems. (Y. Yamamoto, IEEE J. Quantum Electron., Vol. QE-16, pp. 1073-1081 (1980)). Recent reports have not confirmed this prediction, but have demonstrated net gain (see, for example, G. Eisenstein et al, Electron. Lett., Vol. 21, pp. 1076-1077 (1985)). Dual amplifier systems have been demonstrated both with direct detection (I. W. Marshall et al, Electron. Lett., Vol. 22, pp. 253-254, (1986)) and coherent detection (N. A. Olsson, Electron. Lett., Vol. 21, pp. 1085-1087 (1985)) with net amplifier gains of 26 dB and 21 dB, respectively. In a single amplifier phase shift keying (PSK) heterodyne experiment, a net amplifier gain of 8 dB was achieved (R. C. Steel et al, Electron. Lett., Vol. 23, pp. 296-297 (1987)). However, using direct detection, the longest transmission distance, 171 km at 1.2 Gb/s, has been achieved by prior workers in a system which does not use optical amplifiers (M. Shikada et al, IOOC/ECOC, Venice, Italy, pp. 49-52 (1985)). Similarly, using coherent detection, the longest transmission distance did not use optical amplifiers and required unusually low loss fiber to attain 290 km at 400 Mbls (K. Iwashita et al, Electron. Lett., Vol. 22, pp. 791-792 (1986)).